High speed pagewidth inkjet printer

ABSTRACT

An inkjet printer with a pair of inkjet printheads, each having an array of nozzles for ejecting ink, each nozzle having a corresponding nozzle in the array of the other printhead; wherein, defective nozzles in both of the arrays are identified and the printheads are paired such that the corresponding nozzles in each of the arrays are not both defective.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.10/269,999 filed Oct. 15, 2002, which is a continuation of Ser. No.09/505,151 filed Feb. 15, 2000, now issued as U.S. Pat. No. 6,454,378,the entire contents of which are herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to a printer. More particularly, the inventionrelates to the characterisation of a printhead assembly to account fornozzle defects.

BACKGROUND TO THE INVENTION

Pagewidth printheads have the advantage of being able to print rapidlybut are constituted by a very large number of nozzles. Should any ofthese nozzles be defective, an inadequate print quality may result.

Likewise, the use of an ink transfer roller between the nozzles and thepaper can be advantageous, however, it must be quickly and effectivelycleaned if print quality is to be maintained.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided aninkjet printer for printing sheets of media, the printer comprising:

-   -   A pair of inkjet printheads, each having an array of nozzles for        ejecting ink, each nozzle having a corresponding nozzle in the        array of the other printhead; wherein,    -   defective nozzles in both of the arrays are identified and the        printheads are paired such that the corresponding nozzles in        each of the arrays are not both defective.

According to a related aspect there is provided a cleaning assembly foran inkjet printer having an inkjet printhead with an array of nozzlesfor ejecting ink, and,

-   -   a transfer roller for receiving ink ejected from the nozzles and        transferring it to a sheet of media; the cleaning station        comprising:    -   a first and second cleaning members for engaging the transfer        roller after ink has been transferred to the sheet of media but        prior to receiving ink from the array of nozzles; wherein,    -   the first cleaning member has a porous member for engaging the        transfer roller to remove ink and the second cleaning member has        an elongate resilient member for engaging the transfer roller        after the first cleaning member for removing residual ink.

Preferably, the first cleaning member is an elongate sponge thatcontacts the transfer roller along its entire length. In a furtherpreferred form, the second cleaning member is an elongate elastomericwiper. In a particularly preferred form, the drying equipment for aidingdrying of a printed image on the sheet of media comprises a feed pathalong which the web of print media containing the printed image is fedafter printing of the image on the print media;

-   -   a drive means for driving the web along the feed path at a        predetermined rate; and    -   opposed IR lamps mounted on either side of the feed path.    -   a drive means for driving the web along the feed path at a        predetermined rate; and    -   opposed IR lamps mounted on either side of the feed path.

In a related aspect, there is provided a method of characterising aprinthead assembly for a printer, the method including

-   -   matching a pair of printheads, each printhead including a        plurality of inkjet nozzles constructed using        microelectromechanical techniques, such that no paired nozzles        on the pair of printheads are both defective;    -   determining which nozzle of a pair is to be used and generating        data relating to the nozzle to be used; and    -   encoding said data and associating said encoded data with the        printhead assembly.

The printheads may be pagewidth printheads. In this specification,unless the context clearly indicates otherwise, the term “pagewidthprinthead” is to be understood as a printhead having a printing zonethat prints one line at a time on a page, the line being parallel eitherto a longer edge or a shorter edge of the page. The line is printed as awhole as the page moves past the printhead and the printhead isstationary, ie it does not raster or traverse the page.

The method may include forming a defect list, which may be in the formof a characterisation vector, of the generated data and storing thevector in a manufacturing database.

Further, the method may include indexing the characterisation vectorwith an identification device of the assembly. More particularly, thecharacterisation vector associated with a particular printhead may bestored in the manufacturing database and indexed by a serial number ofthe printhead.

Further, the method may include encoding the vector in a readable formatand applying it to a cartridge of the assembly. Preferably, thecharacterisation vector is recorded as a barcode on the cartridge.

Then, when the cartridge is installed in the printer, the method mayinclude retrieving the vector from the barcode and writing the vector toa memory means of a printer controller of the printer. The memory meansmay be a flash memory of the printer controller.

If the cartridge is replaced in the field, a new characterisation vectormay be downloaded remotely from the manufacturing database to theprinter controller via a network interface of the printer controllerusing the barcode of the new cartridge.

According to another aspect, there is provided a printhead assemblywhich includes

-   -   a pair of matched printheads, each printhead including a        plurality of inkjet nozzles constructed using        microelectromechanical techniques, the printheads being matched        so that no paired nozzles of the pair of printheads are both        defective; and    -   encoded data relating to a defect list associated with the        printheads, the defect list providing data relating to which        nozzle of each pair of matched nozzles of the pair of printheads        is to be used.

The defect list, which may be in the form of a characterisation vector,may be associated with an identification device of the assembly and maybe stored in a manufacturing database.

The printhead assembly may include a printhead cartridge, thecharacterisation vector, in its encoded format, being applied to thecartridge to be readable by a printer when the cartridge is installed inthe printer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described by way of example with reference to theaccompanying drawings in which,

FIG. 1 shows a plan view of a printer, in accordance with the invention;

FIG. 2 shows a front view of the printer;

FIG. 3 shows a side view of the printer;

FIG. 4 shows a schematic, sectional front view of the printer;

FIG. 5 shows a schematic, sectional plan view of the printer;

FIG. 6 shows, on an enlarged scale, a schematic, sectional front view ofpart of the printer;

FIG. 7 shows an enlarged front view of a central section of the printer;

FIG. 8 shows a three-dimensional view of a print engine arrangement ofthe printer;

FIG. 9 a shows a three-dimensional top view of an ink cartridge of theprinter;

FIG. 9 b shows a three-dimensional bottom view of the ink cartridge;

FIG. 10 shows a diagrammatic representation of document data flow in theprinter;

FIG. 11 shows a block diagram of the printer controller architecture;

FIG. 12 shows a block diagram of one embodiment of the print enginecontroller architecture; and

FIG. 13 shows a block diagram of another embodiment of the print enginecontroller architecture.

DETAILED DESCRIPTION OF THE DRAWINGS

1 S-Print Overview

The invention will be described with reference to a high-speed duplexnetwork color printer intended for high-volume office use. It features2000-sheet motorized paper trays, 120 page-per-minute operation, and1600 dpi photographic-quality output. We refer to the printer as the“S-print” and we shall refer to it as such or as the printer below.

With 20 times the speed of the best network color laser printers, and 4times the speed of the best network monochrome laser printers, S-printeffectively targets the $40 billion desktop laser printer market. Withits high performance and photographic-quality output, it also competesagainst offset printing for print runs smaller than 5000 copies.

S-print accommodates A4/Letter sized media and, with a tray adaptor,A3/Tabloid sized media. It achieves simultaneous high quality andperformance using full-color page-width 1600 dpi microelectromechanicalinkjet (Memjet) printheads.

S-print uses an embedded DSP-based raster image processor (RIP) torasterize Postscript and PCL page descriptions at high speed. Thestandard RIP uses a single DSP, but up to three additional DSP modulescan be plugged in to increase performance.

The RIP compresses and stores the rasterized page images on an internalhigh-capacity hard disk. While simple page descriptions are rasterizedat the full 120 ppm printing rate, more complex page descriptions maytake longer. Pre-rasterized documents retrieved from the internal harddisk are always printed at the full 120 ppm printing rate. Any documentcan be “printed” to the hard disk, i.e. rendered and stored on the harddisk, for later high-speed retrieval.

Users can walk up to an S-print, select locally-stored documents on itscolor LCD, and print them immediately, without ever going near aworkstation. Documents printed in this way always print at the full 120ppm rate. The standard 14 GB internal hard disk stores over 6000image-intensive pages. Because of its walk-up capability and high speed,S-print is likely to displace many uses of short-run offset printing.

S-print uses duplexed printheads for simultaneous double-sided printing.During the pilot phase of Memjet printhead manufacturing when theprinthead defect density is still potentially high, each printhead isreplicated to achieve 2:1 nozzle redundancy. This allowsfactory-detected defective nozzles to be bypassed, and so maximisesprinthead yield. A pair of custom print engine controllers expand,dither and print page images to the duplexed printheads in real time.

Apart from custom print engine controllers and Memjet printheads,S-print is built using standard off-the-shelf electronic components.

2 Printer Mechanics

S-print is designated generally by the reference numeral 10 andcomprises a housing 12 having a central section 14 (FIGS. 1 and 2). Ahinged tray housing 16 projects from each side of the central section 14(FIG. 3).

Ink cartridges 18, which will be described in greater detail below, aremounted on top of the central section 14 to be readily accessible.

A front face 20 of the central section 14 houses a display 22. Thedisplay 22, which will be described in greater detail below withreference to FIG. 7 of the drawings, is a full color LCD user interface.

Referring now to FIG. 4 of the drawings, a schematic front view ofS-print 10 is shown.

The housing 12 is constructed around a box chassis 24. Print engines 26are centrally located in the central section 12. The print engines 26will be described in greater detail below with reference to FIG. 6 ifthe drawings.

On either side of the central section 14, and projecting outwardlytherefrom is one of the paper tray housings 16. Each paper tray housing16 has a bottom or side hinged door 28. A platen 30 is located in eachpaper tray housing 16 for supporting a load of papers. An operativelyinner end of each platen 30 has a guide roller 32 which is received in avertically extending channel 34 for guiding vertical movement of theplaten 30. Each platen 30 is driven by a motor 36. The motor 36 drives asprocket 38. A second sprocket 40 is mounted vertically below thesprocket 38. The sprockets 38 and 40 are interconnected by an endlesschain 42 which drives vertical movement of the platen 30.

A first motor assembly 44 is arranged upstream of the print engines 26for feeding print media, in the form of a sheet of paper, between theprint engines 26. A second motor assembly 46 is arranged downstream ofthe print engines 26 for drawing the sheet of paper from the printengines 26 after printing.

The platens 30 rise and descend according to the volume of paper in theinput and output stacks.

A compact power supply 48 is arranged below the print engines 26 as is a14 GB hard disk drive (HDD) 50 and controlling circuitry 52.

S-print 10 prints the long edge of the paper to achieve a compact formfactor and a minimised footprint.

As illustrated more clearly in FIG. 5 of the drawings, a sheet of paperto be fed to the print engines 26 is guided by pick-up rollers 54arranged upstream of the print engines 26 in the paper path. Spike wheelrollers 56 grip a leading edge of the paper, after printing, for guidingthe printed paper to a paper tray housing 16 located downstream of theprint engines 26 in the paper path. The pick up rollers 54 are driven bya motor assembly 44. Similarly, the spike wheel rollers 56, which arearranged in vertically spaced pairs, are driven by a further motorassembly 46.

Also, as illustrated in FIG. 5 of the drawings, S-print 10 is a fourcolor printer having a cyan ink cartridge 58, a magenta ink cartridge60, a yellow ink cartridge 62 and a black ink cartridge 64. The inkcartridges 58, 60, 62 and 64 feed ink via hoses 66 to the print engines26.

A molding 72 (FIG. 6) to which the ink cartridges 58 to 64 are attachedis hingedly secured to the remainder of the central section 14 of thehousing 12 to reveal an upper part of a chassis 74 of the print engines26. This upper part 74 can be pivoted about pivot pin 76 to enableaccess to be gained to an upper print engine 26.1. It is to be notedthat the upper print engine 26.1 is secured to the part 74 so that, whenthe part 74 is pivoted, access can be gained to a lower print engine26.2 as well as drying infrared lamp 78.

These infrared lamps 78 are mounted on paper guides 80 which guide asheet of paper 82 between the print engines 26.

The straight paper path allows the paper 82 to be fed at high speed pastprintheads of the print engines 26.

The two print engines 26.1 and 26.2 are mounted together in anadjustable assembly. As described above, the upper print engine 26.1 canbe pivoted upwards to allow access to paper jams and to the lower printengine 26.2 and the infrared drying lamps 78.

As schematically illustrated in FIG. 6 of the drawings, the relevant inkcartridges 58 to 64 are snap fits on the top molding 72.

Each ink cartridge 58 to 64 comprises moldings 86 defining a reservoir90. The reservoir 90 is, in use, in fluid flow communication with afixed reservoir 88 defined in a molding 84 on top of the print engines26.

The reservoir 90 is in communication with the fixed reservoir 88 via apassage 92. A pin 94 projects through the passage 92 and is engaged by aball 96 of the ink cartridge 58 to 64. The ball 96 is urged intoengagement with the pin 94 by means of a spring 98. The pin 94 has acollar or flange 100 at its operatively lower end, i.e. that end withinthe reservoir 88. When the collar 100 is urged off its seat, ink canflow from the reservoir 90 into the reservoir 88.

The reservoir 88 serves to provide an early warning to replace the inkcartridge and makes contact with an embedded QA cartridge chip. Eachreservoir 88 connects via the hoses 66 to the printheads of the printengines 26.

Referring now to FIG. 7 of the drawings the front panel 20 of S-print 10is shown in greater detail. As described above, the front panel 20contains a color LCD interface 22. A power switch 102 is arranged belowthe interface 22.

A keypad 104 is also arranged on the front panel 20. The keypad 104allows the desired number of copies to be entered. Documents to beprinted locally, i.e. at the printer 10 can also be selected by anidentification number by means of the keypad 104. It will be appreciatedthat this can be quicker than scrolling through stored documents ifthere are many such documents.

The LCD interface 22 includes four changeable function buttons 106 fornavigating the interface 22.

A print button 108 and a stop button 110 are also arranged on the frontpanel 20 adjacent to the keypad 104.

Referring to FIG. 8 of the drawings the print engines 26 are discussedin greater detail. As described above, an upper print engine 26.1 and alower print engine 26.2 are provided. S-print 10 uses duplex printengines 26.1 and 26.2 for simultaneous double-sided printing.

It is to be noted that each print engine 26.1 and 26.2 uses twoprintheads 112 (only one of which is shown in respect of the printengine 26.1). The two printheads 112 are provided to achieve a 2:1nozzle redundancy. This allows factory-detected defective nozzles to bebypassed and so maximises the printhead yield.

The printheads 112 print on to a transfer roller 114. The roller 114 isrotatably driven by a co-axially arranged motor 116. Ink deposited on asurface of the roller 114 is, in turn, deposited on the paper 82 duringthe printing process. In addition, when the printheads 112 areinoperative, the roller 116 is urged into engagement with the printheads112 for inhibiting evaporation of ink in reservoirs 118 in eachprinthead assembly 112.

Each print engine 26 includes a cleaning station 120. The cleaningstation 120 includes a wiper 122 of a resiliently flexible, elastomericmaterial and a sponge 124 arranged upstream of the wiper 122 so that thesponge 124 removes ink from the transfer roller 114 before the wiper 122wipes ink from the transfer roller 114.

Movement of the roller 114 into and out of engagement with the printheadassemblies 112 is controlled by a solenoid arrangement 126.

A three dimensional top view of one of the ink cartridges 60 is shown inFIG. 9 a of the drawings with a three dimensional bottom view of thecartridge 60 being shown in FIG. 9 b of the drawings. The cartridge 60comprises the moldings 86 which engages the molding 84 in the centralsection 14 of the housing 12 of S-print 10. A QA chip 128 is shown on abottom surface 130 of the ink cartridge 60 in FIG. 9 b of the drawings.

As shown in FIG. 6 of the drawings, but not shown in FIG. 9 a or 9 b ofthe drawings, each upper molding 86 houses the sprung ball 96 which isheld captive against the lower molding 84 to provide a main seal to thecartridge 60. A secondary hydrophobic, elastomeric seal 132 is providedat an entry port on a lower surface 130 of the cartridge 60.

As described above, the cartridge 60 connects to the print engines 26via the printer ink reservoir 88 by means of the pin 94.

Tortuous air channels 134 (FIG. 9 a) are provided at the top of thecartridge 60 under the color label 136. The four ink cartridges 58 to 64are keyed by plastic protrusions to prevent any incorrect insertion ororientation of the cartridges 58 to 64. Also, it is to be noted that theblack cartridge 64 holds twice the volume of the other cartridges due tothe greater use of black ink.

3 Memjet-Based Printing

A Memjet printhead 112 produces 1600 dpi bi-level CMYK (Cyan, Magenta,Yellow, blacK). On low-diffusion paper, each ejected drop forms analmost perfectly circular 22.5 micron diameter dot. Dots are easilyproduced in isolation, allowing dispersed-dot dithering to be exploitedto its fullest. Since the Memjet printhead 112 is the width of the pageand operates with a constant paper velocity, the four color planes areprinted in good registration, allowing accurate dot-on-dot printing.Since there is consequently no spatial interaction between color planes,the same dither matrix is used for each color plane. Dot-on-dot printingminimizes ‘muddying’ of midtones caused by inter-color bleed.

A page layout may contain a mixture of images, graphics and text.Continuous-tone (contone) images and graphics are reproduced using astochastic dispersed-dot dither. Unlike a clustered-dot (oramplitude-modulated) dither, a dispersed-dot (or frequency-modulated)dither reproduces high spatial frequencies (i.e. image detail) almost tothe limits of the dot resolution, while simultaneously reproducing lowerspatial frequencies to their full color depth, when spatially integratedby the eye. A stochastic dither matrix is carefully designed to be freeof objectionable low-frequency patterns when tiled across the image. Assuch its size typically exceeds the minimum size required to support aparticular number of intensity levels (e.g. 16×16×8 bits for 257intensity levels). S-print 10 uses a dither volume of size 64×64×3×8bits. The volume provides an extra degree of freedom during the designof the dither by allowing a dot to change states multiple times throughthe intensity range (rather than just once as in a conventional dithermatrix).

Human contrast sensitivity peaks at a spatial frequency of about 3cycles per degree of visual field and then falls off logarithmically,decreasing by a factor of 100 beyond about 40 cycles per degree andbecoming immeasurable beyond 60 cycles per degree. At a normal viewingdistance of 12 inches (about 300 mm), this translates roughly to 200-300cycles per inch (cpi) on the printed page, or 400-600 samples per inchaccording to Nyquist's theorem.

In practice, contone resolution above about 300 ppi is of limitedutility outside special applications such as medical imaging. Offsetprinting of magazines, for example, uses contone resolutions in therange 150 to 300 ppi. Higher resolutions contribute slightly to colorerror through the dither.

Black text and graphics are reproduced directly using bi-level blackdots, and are therefore not antialiased (i.e. low-pass filtered) beforebeing printed. Text is therefore supersampled beyond the perceptuallimits discussed above, to produce smoother edges when spatiallyintegrated by the eye. Text resolution up to about 1200 dpi continues tocontribute to perceived text sharpness (assuming low-diffusion paper, ofcourse).

S-print 10 uses a contone resolution of 320 ppi (i.e. 1600÷5), and ablack text and graphics resolution of 1600 dpi.

4 Document Data Flow

Document transmission and document rasterization are decoupled to shieldthe user from interactions between the size and complexity of thedocument, and the memory capacity and RIP performance of S-print 10.This is achieved by storing each document's page description language(PDL) file on the internal hard disk 50.

Because of the high resolution of the Memjet printhead 112, each pagemust be printed at a constant speed to avoid creating visible artifacts.This means that the printing cannot be varied to match the input datarate. Document rasterization and document printing are thereforedecoupled to ensure the printhead 112 has a constant supply of data. Apage is never printed until it is fully rasterized. This is achieved bystoring a compressed version of each rasterized page image on theinternal hard disk 50.

This decoupling also allows the RIP to run ahead of the printer 10 whenrasterizing simple pages, buying time to rasterize more complex pages.

The user indicates whether a document is to be stored permanently on thehard disk 50, printed, or both. So long as there is disk spaceavailable, the pages of transient documents are also cached on the disk50 until printed. This is particularly efficient when multiple copies ofcomplex documents are being printed. This so-called electronic collationalso obviates the need for an external collating mechanism, since eachcopy of a document is printed in its entirety before the next copy.

Because contone color images are reproduced by stochastic dithering, butblack text and line graphics are reproduced directly using black dots,the compressed page image format contains a separate foreground bi-levelblack layer and background contone color layer. The black layer iscomposited over the contone layer after the contone layer is dithered.

FIG. 10 shows the flow of an S-print document from network to printedpage.

At 320 ppi, an A4/Letter page of contone CMYK data has a size of 38 MB.Using lossy contone compression algorithms such as JPEG, contone imagescompress with a ratio up to 10:1 without noticeable loss of quality,giving a compressed page size of 3.8 MB.

At 1600 dpi, an A4/Letter page of bi-level data has a size of 30 MB.Coherent data such as text compresses very well. Using lossless bi-levelcompression algorithms such as Group 4 Facsimile, ten-point textcompresses with a ratio of about 20:1, giving a compressed page size of1.5 MB.

Once dithered, a page of CMYK contone image data consists of 120 MB ofbi-level data. Using lossless bi-level compression algorithms on thisdata is pointless precisely because the optimal dither isstochastic—i.e. since it introduces hard-to-compress disorder.

The two-layer compressed page image format therefore exploits therelative strengths of lossy JPEG contone image compression and losslessbi-level text compression. The format is compact enough to bestorage-efficient, and simple enough to allow straightforward realtimeexpansion during printing.

Since text and images normally do not overlap, the normal worst-casepage image size is 3.8 MB (i.e. image-only), while the normal best-casepage image size is 1.5 MB (i.e. text-only). The absolute worst-case pageimage size is 5.3 MB (i.e. text over image). Assuming a third of anaverage page contains images, the average page image size is 2.3 MB. Thestandard 14 GB internal hard disk therefore holds over 6000 such pages.

5 Printer Controller Architecture

The S-print 10 printer controller consists of a controlling processor138 (FIG. 11), various peripheral controllers 140, 142 and 144, a rasterimage processor (RIP) DSP farm 146, and duplexed page expansionprocessors 148. These components are discrete and communicate via ashared bus 150 and a shared 64 MB memory 152.

The controlling processor 138 handles communication with the network viaan Ethernet controller 140, controls the internal hard disk 50 via theSCSI controller 142 and controls the LCD 22 via the LCD controller 144.The controller 138 also controls the paper transport, handles inkcartridge authentication and ink monitoring, and feeds and synchronizesthe RIP and the print engine controllers 148. It consists of amedium-performance general-purpose microprocessor. Its associatedperipheral controllers include a 10/100Base-T Ethernet controller (140),a SCSI disk controller (142), and a color TFT LCD controller (144).Optional controllers include an IEEE 1394 (Firewire) controller and aUSB 2.0 controller for high-speed point-to-point communication with aworkstation or server.

The RIP DSP farm 146 rasterizes and compresses page descriptions toS-print's compressed page format. The DSP farm 146 consists of betweenone and four general-purpose high-performance DSPs. Each additional DSPcomes as a field-installable plug-in module.

Each print engine controller 148 expands, dithers and prints page imagesto its associated replicated printhead 112 in real time (i.e. at 60ppm). The duplexed print engines 26 print both sides of the pagesimultaneously (i.e. at 120 ppm).

The printer controller's flash memory 154 holds the software for boththe processor 138 and the DSPs 146. This is copied to main memory 152 atboot time. The flash memory 154 also holds the defect lists for the tworeplicated printheads 112. These are copied to the print enginecontrollers 148 at boot time.

5.1 Detailed Document Data Flow

The main processor 138 receives the document's page description language(PDL) file and stores it on the internal hard disk 50. It then runs theappropriate RIP software on the DSPs 146.

The DSPs 146 rasterize each page description and compress the rasterizedpage image. The main processor 138 stores each compressed page image onthe hard disk 50. The simplest way to load-balance multiple DSPs 146 isto let each DSP 146 rasterize a separate page. The DSPs 146 can alwaysbe kept busy since an arbitrary number of rasterized pages can, ingeneral, be stored on the internal hard disk 50. This strategy can leadto poor DSP utilization, however, when rasterizing short documents.

The main processor 138 passes back-to-back page images to thecontrollers 148 of the duplexed print engines 26. Each print enginecontroller 148 stores the compressed page image into its local memory,and starts the page expansion and printing pipeline. Page expansion andprinting is pipelined because it is impractical to store a 120 MBbi-level CMYK image in memory.

The first stage of the pipeline expands the JPEG-compressed contone CMYKlayer. The second stage, in parallel with the first, expands the Group 4Fax-compressed bi-level black layer. The third stage dithers the contoneCMYK layer, and composites the bi-level black layer over the resultingbi-level CMYK layer. The fourth stage prints the bi-level CMYK data viathe printhead interface which controls the Memjet printhead 112.

The main processor 138 streams compressed page images from the hard disk50 to the print engine controllers 148 at the required 120 ppm rate(i.e. 4.6 MB/s on average, or 10.6 MB/s worst-case). TABLE 1 Printengine controller page image and FIFO data flow input input outputoutput input output process format window format window rate ratereceive — — JPEG 1 —  3.8 MB/s (10 Mp/s) contone stream receive — —G4Fax 1 —  1.5 MB/s (250 Mp/s) bi-level stream expand JPEG — 32-bit 8 3.8 MB/s (10 Mp/s)   38 MB/s (10 Mp/s) contone stream CMYK expand G4Fax— 1-bit K 1  1.5 MB/s (250 Mp/s)   30 MB/s (250 Mp/s) bi-level streamdither 32-bit 1 —^(a) —   38 MB/s (10 Mp/s^(b)) — CMYK composite 1-bit K1 4-bit 1   30 MB/s (250 Mp/s)  120 MB/s (250 Mp/s) CMYK print 4-bit 24,1^(c) — —  120 MB/s (250 Mp/s) — CMYK —  193 MB/s  193 MB/s 387 MB/s^(a)dither combines with composite, so there is no external data flowbetween them^(b)320 ppi

1600 dpi (5 × 5 expansion)^(c)Needs a window of 24 lines, but only advances 1 line

The print engine data flow is summarized in Table 1. The aggregatetraffic to/from memory is 387 MB/s, all but 5.3 MB/s of which relates tothe FIFOs.

Each stage communicates with the next via a FIFO. Each FIFO is organizedinto lines, and the minimum size (in lines) of each FIFO is designed toaccommodate the output window (in lines) of the producer and the inputwindow (in lines) of the consumer. The inter-stage memory FIFOs aredescribed in Table 2. TABLE 2 Print engine controller local memory FIFOsnumber of FIFO format and line size lines FIFO size contone 32-bitinterleaved CMYK  8 × 2 = 16 240 KB CMYK (320 ppi × 11.7″ × 32 = 15.0KB) bi-level K 1-bit K  1 × 2 = 2  5 KB (1600 dpi × 11.7″ × 1 = 2.3 B)bi-level 4-bit planar odd/even CMYK 24 + 1 = 25 229 KB CMYK (1600 dpi ×11.7″ × 4 = 9.1 KB) 474 KB

Because the two printheads 112 of each redundant printhead pair areseparated by about 8 mm on the transfer roller (or about 500 printedlines at 1600 dpi), an additional 500 lines of bi-level CMYK must bebuffered between the ditherer/compositor unit 176 and the printheadinterface 178. This in turn translates to about 4.5 MB of additionalFIFO memory, or about 5 MB of FIFO memory in total.

The need for this additional FIFO memory can be eliminated by expandingeach page image twice in parallel, i.e. once each for the two printheads112 of each redundant printhead pair, staggered to match the physicalseparation of the printheads. This is most easily done by replicatingthe print engine controller 148 for each printhead 112. Replication isparticularly relevant in the case of the pipelined (as opposed toshared-memory) print engine controller 148 described below, where theprovision of 5 MB of on-chip FIFO memory is impractical.

It is also possible to run each print engine controller 148 at twice therate so that it can expand each page image twice in the time it takes toprint the page image once.

Whenever each page image is expanded twice in parallel, FIFO memory mustbe doubled to about 1 MB.

5.2 Print Engine Controller Architecture

The print engine controller 148 is implemented as a single custom chip.

There are two architectural variants of the print engine controller 148.The shared-memory version, illustrated in FIG. 12, uses a local off-chipRDRAM 156 to support the aggregate memory bandwidth required by pageexpansion and printing. The pipelined version, illustrated in FIG. 13,uses dedicated on-chip FIFOs 158, 159, 160.

The shared-memory print engine controller 148 consists of ageneral-purpose processor 162, a high-speed Rambus interface 164 to theoff-chip RDRAM 156, a small program ROM 166, a DMA controller 168, andan interface 170 to the printer controller bus 150.

Both print engine controllers' page expansion and printing pipelineconsists of a standard JPEG decoder 172, a standard Group 4 Fax decoder174, a custom ditherer/compositor unit 176, and a custom interface 178to the Memjet printheads 112.

The ditherer/compositor unit 176 and the printhead interface 178 aredescribed in greater detail in co-pending U.S. patent application Ser.No. 09/436,744 which is incorporated herein by reference.

In the shared-memory version, the FIFOs are located in the dedicatedoff-chip RDRAM 156, and all inter-stage communication is controlled bythe local processor via the DMA controller 168. In the pipelinedversion, the FIFOs 158, 159, 160 are on-chip, and the stages areself-synchronizing.

In the shared-memory version, the decoders 172, 174 obtain page datafrom the main processor 138 via the local memory. In the pipelinedversion, the decoders 172, 174 obtain page data directly from the mainprocessor 138 over the printer controller bus 150.

When several print engine controllers 148 are used in unison, such as ina duplexed configuration, they are synchronized via a shared line syncsignal on line 180. Only one print engine controller 148, selected viaan external master/slave pin 182, generates the line sync signal ontothe shared line 180.

5.3 Printhead Timing

Each print engine controller 148 prints an A4/Letter page in one second.Since S-print 10 uses a 12″ printhead 112 to print the long dimension ofthe page (11.7″), the short dimension of the page (8.5″) needs to passthe printhead 112 in one second. At 1600 dpi, this equates to a 13.6 KHzline rate. This is well within the operating frequency of the Memjetprinthead 112, which in the current design exceeds 30 KHz.

5.4 Printhead Characterization

Each redundant 12″ print engine 26 contains two complete 12″ printheads112, i.e. 76,800 nozzle pairs, characterized and matched so that nopaired nozzles are both defective.

Printhead defects are either characterized and matched one segment at atime, or after the entire printhead has been built. In the former casenozzles are tested before integration with the ink path, and so aretested without ink. In the latter case nozzles are tested afterintegration with the ink path, and so are tested with ink. Segment-wisecharacterization gives a higher yield, but at a higher testing cost.Segment-wise characterization is therefore only preferable toprinthead-wise characterization when defect densities are still high.

The defect list associated with a redundant printhead is stored in themanufacturing database, indexed by the printhead's serial number andrecorded as a barcode on its cartridge. When the printhead cartridge isfinally inserted into a printer during manufacture, the defect list isretrieved using the barcode, and is written to the flash memory of theprinter's embedded printer controller.

If the printhead cartridge is replaced in the field, then a new defectlist is downloaded remotely from the manufacturing database to theprinter controller via its network interface, using the new printheadcartridge's barcode.

The defect list associated with each redundant printhead pair is copiedfrom the printer controller's flash memory 154 to the correspondingprint engine controller 148 at boot time. During printing, each printengine controller 148 consults its defect list to determine which nozzleof each nozzle pair to direct data to. When one nozzle of a nozzle pairis defective, the print engine controller 148 directs data to the othernozzle. Printhead characterization and matching ensures that the twonozzles of a nozzle pair are never both defective.

1. An inkjet printer for printing sheets of media, the printercomprising: a pair of inkjet printheads, each having an array of nozzlesfor ejecting ink, each nozzle having a corresponding nozzle in the arrayof the other printhead; wherein, defective nozzles in both of the arraysare identified and the printheads are paired such that the correspondingnozzles in each of the arrays are not both defective.
 2. An inkjetprinter according to claim 1 further comprising a transfer roller forreceiving ink from the nozzles in each of the arrays, and transferringit to a sheet of the media.
 3. An inkjet printer according to claim 2further comprising a cleaning station for engaging the transfer rollerafter ink has been transferred to the sheet of media but prior toreceiving ink from the array of nozzles, the cleaning station havingfirst and second cleaning members, the first cleaning member having aporous member for engaging the transfer roller to remove ink and thesecond cleaning member having an elongate resilient member for engagingthe transfer roller after the first cleaning member for removingresidual ink.
 4. An inkjet printer according to claim 3 furthercomprising drying equipment for aiding drying of a printed image on thesheet of media.
 5. An inkjet printer according to claim 4 wherein thedrying equipment for aiding drying of a printed image on the sheet ofmedia comprises a feed path along which the web of print mediacontaining the printed image is fed after printing of the image on theprint media; a drive means for driving the web along the feed path at apredetermined rate; and opposed IR lamps mounted on either side of thefeed path. a drive means for driving the web along the feed path at apredetermined rate; and opposed IR lamps mounted on either side of thefeed path.
 6. An inkjet printer according to claim 3 wherein the firstcleaning member is an elongate sponge that contacts the transfer rolleralong its entire length, and the second cleaning member is an elongateelastomeric wiper.
 7. An inkjet printer according to claim 1 wherein thedefective nozzles identified in each array are recorded as acharacterisation vector for each individual printhead respectively suchthat if one of the printheads in the pair of printhead is replaced, areplacement printhead is selected to have a compatible characterisationvector that will ensure that corresponding nozzles in the pair ofprintheads will not both be defective.
 8. An inkjet printer according toclaim 1 wherein the printheads are pagewidth printheads.